aimgr repo sync fails silently on network error and does not surface actionable error message

[hk9890]

Bug

When aimgr repo sync fails due to a network error (e.g. cannot reach GitHub), it fails silently with only a generic warning:

⚠️  aimgr repo sync failed - proceeding with existing repository
   Resources may be outdated. Try: aimgr repo sync

No error detail is shown — what failed, which source, or why (timeout, DNS, auth, etc.).

Steps to Reproduce

1. Lose or restrict network access to https://github.com/anthropics/skills
2. Run aimgr repo sync

Actual Behavior

Silent failure with a generic warning. The user has no way to distinguish:

• Transient network hiccup
• Permanent connectivity issue
• Auth failure
• Wrong URL configured

Expected Behavior

• Show which source(s) failed and why (e.g. [1/4] anthropics: connection timeout)
• Exit with a non-zero code if any source fails
• Or at minimum surface the underlying error message

Impact

Users cannot diagnose the root cause and may assume their entire configuration is broken rather than a single remote source being temporarily unreachable.